Zoom lens system, imaging device and camera

ABSTRACT

A zoom lens system comprising a first lens unit having negative power, a second lens unit having positive power and a third lens unit having positive power, wherein in zooming from a wide-angle limit to a telephoto limit, the lens units move respectively along an optical axis in such a manner that an interval between the first lens unit and the second lens unit decreases while an interval between the second lens unit and the third lens unit changes so that variable magnification is achieved, the first lens unit comprises one object side negative lens element and one image side positive lens element with a convex surface facing the object side, which have an aspheric surface, and the conditions: n12&gt;1.88 and ν12&lt;26 (n12 and ν12 are refractive index and Abbe number, respectively, of the image side positive lens element of the first lens unit) are satisfied.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No.11/605,308, filed on Nov. 29, 2006 now U.S. Pat. No. 7,453,648 and isbased on Japanese application No. 2005-347204 filed in Japan on Nov. 30,2005, the entire contents of each of which are hereby incorporated byreference.

BACKGROUND

1. Technical Field

The present invention relates to a zoom lens system, an imaging deviceand a camera. In particular, the present invention relates to: a zoomlens system that has a high resolution, high capability of compensatingcurvature of field, and a short overall optical length at the time ofnon-use; an imaging device employing this zoom lens system; and a cameraemploying this imaging device.

2. Description of the Background Art

In the prior art, a large number of optical instruments have beendeveloped that form an image of a photographic object onto an imagesensor through a lens and then acquire the object image as an image.Recently, products such as digital still cameras and digital videocameras are spreading. Then, with the increase in the number of users,desire on these products is also growing. Among various types of theseproducts, optical instruments having a zoom ratio of approximately threeare comparatively small and still have an optical zoom function. Thus,these types are spreading remarkably widely as digital cameras ofcompact type or stylish type.

In the digital cameras of compact type, for the purpose of the propertyof easy carrying, further size reduction of the instruments is desired.In order to achieve the further size reduction of the digital cameras,the lens arrangement need be adopted such that the overall opticallength (the distance measured from the top of the most object side lenssurface of the entire lens system to the image surface) at the time ofnon-use should be reduced while lens elements that extend out relativeto the main body by means of a multi-stage lens barrel at the time ofuse could be accommodated into the main body.

Meanwhile, as zoom lens systems suitable for digital still cameras ofcompact type, a large number of zoom lens systems of three-unitconstruction have been proposed that, for example, in order from theobject side to the image side, comprise a first lens unit havingnegative optical power, a second lens unit having positive opticalpower, and a third lens unit having positive optical power.

In such a zoom lens system of three-unit construction, in zooming(magnification change) from a wide-angle limit to a telephoto limit, theair space between the first lens unit and the second lens unit decreasesmonotonically, while the air space between the second lens unit and thethird lens unit varies, and while the third lens unit is fixed or moved.

Focus adjustment in the zoom lens system of three-unit construction isperformed by moving the first lens unit or the third lens unit in theoptical axis direction. In particular, from the perspective of sizereduction of the entire optical instrument, in many cases, the focusadjustment is performed using the third lens unit which is less heavy,so that focusing onto the photographic object is achieved ranging fromthe infinity to a short distance. In contrast, when the focus adjustmentis performed using the first lens unit, the first lens unit is largerthan the third lens unit and hence requires a large size motor. Thiscauses a tendency of size increase in the entire optical instrument.

The third lens unit having positive optical power has the effects ofcompensating curvature of field and bringing into a telecentric statethe incident light onto the imaging surface. Further, in many cases, thethird lens unit is constructed from one or two lens elements having asmall outer diameter, and hence can be driven at a high speed using asmall size motor. Thus, when the third lens unit is adopted as a lensunit for focus adjustment, an optical instrument is realized that has areduced size and permits rapid focusing.

The first lens unit and the second lens unit move in parallel to theoptical axis along a cam groove formed in a cylindrical cam. In the camgroove, a groove for zooming and a groove for the time of non-use areconnected to each other. The groove for the time of non-use reduces theinterval between the lens units and moves all the three lens units tothe image sensor side. This configuration reduces the overall opticallength at the time of non-use. In this case, if the thickness of eachlens unit could be reduced, the overall optical length at the time ofnon-use would be reduced further.

As such, in the prior art, design has been performed such that the zoomlens system should have the above configuration where the size isreduced in the part relevant to focus adjustment and in the entire lenssystem at the time of non-use, so that the overall optical length of thedigital still camera has been reduced.

For example, Japanese Laid-Open Patent Publication No. 2005-134746discloses a three-unit zoom lens, in order from the object side to theimage side, comprising: a first lens unit having negative optical powerthat is composed of a negative-powered lens having an aspheric surfaceand a positive-powered lens; a second lens unit having positive opticalpower; and a third lens unit having positive optical power. In thisthree-unit zoom lens, the most object side negative-powered lens of thefirst lens unit is provided with a high refractive index, so that thelens thickness in the periphery part is reduced in a state thatcurvature of field at a wide-angle limit is compensated. This reducesthe thickness of the entire first lens unit and hence the size of theoptical system.

Further, for example, Japanese Laid-Open Patent Publication No.2005-084597 discloses a three-unit zoom lens that, in order from theobject side to the image side, comprises a first lens unit havingnegative optical power, a second lens unit having positive optical powerand provided with a diaphragm, and a third lens unit having positiveoptical power, wherein in magnification change, the first lens unitmoves relatively in a direction approaching to the second lens unit,while the second lens unit monotonically moves to the object side, andwhile the third lens unit moves to the object side and then movesreverse to the image side, and wherein when the object distance isinfinity, the position of the third lens unit at a wide-angle limit islocated on the object side relative to the position at a telephotolimit. In this three-unit zoom lens, a condition is set forth concerningthe focal length of the first lens unit in such a manner that thecompensation of curvature of field and the size reduction of the opticalsystem can be achieved simultaneously.

Nevertheless, in the configuration of the three-unit zoom lens disclosedin Japanese Laid-Open Patent Publication No. 2005-134746, thepositive-powered lens on the image side of the first lens unit has a lowrefractive index and still is a spherical lens. This causes a problem ofinsufficiency in the compensation of curvature of field.

Further, in the configuration of the three-unit zoom lens disclosed inJapanese Laid-Open Patent Publication No. 2005-084597, for the purposeof size reduction, the focal length of the first lens unit is set uprather short. Nevertheless, in this case, although the diameter of thelens can be constructed comparatively small, when the first lens unit iscomposed of two lenses, the optical power becomes excessive in theobject side lens. Further, the thickness of the image side lens alsoincreases for the purpose of compensation of chromatic aberration. Thiscauses a problem of increase in the overall optical length at the timeof non-use.

SUMMARY

An object of the present invention is to provide: a zoom lens systemthat has a high resolution, high capability of compensating curvature offield, and a short overall optical length at the time of non-use; animaging device employing this zoom lens system; and a camera employingthis imaging device.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a zoom lens system for forming an optical image of an object with avariable magnification, in order from the object side to the image side,comprising: a first lens unit having negative optical power; a secondlens unit having positive optical power; and a third lens unit havingpositive optical power, wherein

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit decreaseswhile an interval between the second lens unit and the third lens unitchanges so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens element;and one image side positive lens element with a convex surface facingthe object side,

each of the two lens elements constituting the first lens unit has anaspheric surface, and

the following conditions (1) and (2) are satisfied:n12>1.88  (1)ν12<26  (2)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit, and

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

an imaging device capable of converting an optical image of aphotographic object into an electric image signal and then outputtingthe signal, comprising:

a zoom lens system that forms the optical image of the photographicobject with a variable magnification; and

an image sensor that converts the optical image of the photographicobject formed by the zoom lens system into the electric image signal,wherein

the zoom lens system, in order from the object side serving as thephotographic object side to the image side, comprises: a first lens unithaving negative optical power; a second lens unit having positiveoptical power; and a third lens unit having positive optical power,

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit decreaseswhile an interval between the second lens unit and the third lens unitchanges so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens element;and one image side positive lens element with a convex surface facingthe object side,

each of the two lens elements constituting the first lens unit has anaspheric surface, and

the following conditions (1) and (2) are satisfied:n12>1.88  (1)ν12<26  (2)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit, and

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit.

The novel concepts disclosed herein were achieved in order to solve theforegoing problems in the conventional art, and herein is disclosed:

a camera capable of shooting a photographic object and then outputtingits image as an electric image signal, comprising:

an imaging device including a zoom lens system that forms the opticalimage of the photographic object with a variable magnification, and animage sensor that converts the optical image of the photographic objectformed by the zoom lens system into the electric image signal, wherein

the zoom lens system, in order from the object side serving as thephotographic object side to the image side, comprises: a first lens unithaving negative optical power; a second lens unit having positiveoptical power; and a third lens unit having positive optical power,

in zooming from a wide-angle limit to a telephoto limit, the lens unitsmove respectively along an optical axis in such a manner that aninterval between the first lens unit and the second lens unit decreaseswhile an interval between the second lens unit and the third lens unitchanges so that the variable magnification is achieved,

the first lens unit comprises: one object side negative lens element;and one image side positive lens element with a convex surface facingthe object side,

each of the two lens elements constituting the first lens unit has anaspheric surface, and

the following conditions (1) and (2) are satisfied:n12>1.88  (1)ν12<26  (2)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit, and

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit.

The present invention provides: a zoom lens system that has a highresolution, capability of satisfactory compensation of curvature offield, a reduced thickness of the first lens unit, and a short overalloptical length at the time of non-use; and an imaging device employingthis zoom lens system. The present invention further provides a smalland high performance camera employing this imaging device.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other objects and features of this invention will become clearfrom the following description, taken in conjunction with the preferredembodiments with reference to the accompanied drawings in which:

FIGS. 1A to 1C are configuration diagrams of a zoom lens systemaccording to Embodiment 1 (Example 1);

FIGS. 2A to 2I are longitudinal aberration diagrams of a zoom lenssystem according to Example 1;

FIGS. 3A to 3C are configuration diagrams of a zoom lens systemaccording to Embodiment 2 (Example 2);

FIGS. 4A to 4I are longitudinal aberration diagrams of a zoom lenssystem according to Example 2;

FIGS. 5A to 5C are configuration diagrams of a zoom lens systemaccording to Embodiment 3 (Example 3);

FIGS. 6A to 6I are longitudinal aberration diagrams of a zoom lenssystem according to Example 3;

FIGS. 7A to 7C are configuration diagrams of a zoom lens systemaccording to Embodiment 4 (Example 4);

FIGS. 8A to 8I are longitudinal aberration diagrams of a zoom lenssystem according to Example 4; and

FIG. 9 is a schematic construction diagram of a digital still cameraaccording to Embodiment 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments 1 to 4

FIGS. 1A to 1C are configuration diagrams of a zoom lens systemaccording to Embodiment 1. FIGS. 3A to 3C are configuration diagrams ofa zoom lens system according to Embodiment 2. FIGS. 5A to 5C areconfiguration diagrams of a zoom lens system according to Embodiment 3.FIGS. 7A to 7C are configuration diagrams of a zoom lens systemaccording to Embodiment 4. Each of FIGS. 1A to 1C, 3A to 3C, 5A to 5C,and 7A to 7C shows a zoom lens system in an infinity in-focus condition.FIGS. 1A, 3A, 5A and 7A show the lens construction at a wide-angle limit(the shortest focal length condition: focal length f_(W)). FIGS. 1B, 3B,5B and 7B show the lens construction at a middle position (the middlefocal length condition: focal length f_(M)=√(f_(W)*f_(T))). FIGS. 1C,3C, 5C and 7C show the lens construction at a telephoto limit (thelongest focal length condition: focal length f_(T)).

Each zoom lens system according to Embodiments 1 to 4, in order from theobject side to the image side, comprises: a first lens unit G1 havingnegative optical power; a diaphragm A; a second lens unit G2 havingpositive optical power; and a third lens unit G3 having positive opticalpower. In the zoom lens system according to Embodiments 1 to 4, inzooming from the wide-angle limit to the telephoto limit, the first lensunit G1 moves with locus of a convex to the image side, while the secondlens unit G2 and the diaphragm A monotonically move to the object side,and while the third lens unit G3 moves with changing the interval withthe second lens unit G2. That is, in the zoom lens system according toEmbodiments 1 to 4, in zooming from the wide-angle limit to thetelephoto limit, the lens units move respectively along the optical axisin such a manner that the interval between the first lens unit G1 andthe second lens unit G2 decreases while the interval between the secondlens unit G2 and the third lens unit G3 changes. Further, in each ofFIGS. 1A to 1C, 3A to 3C, 5A to 5C, and 7A to 7C, a straight line drawnon the rightmost side indicates the position of an image surface S. Onits object side, a plane parallel plate P such as an optical low-passfilter, a face plate of an image sensor or the like is provided.

As shown in FIGS. 1A to 1C, in the zoom lens system according toEmbodiment 1, the first lens unit G1, in order from the object side tothe image side, comprises two lens elements consisting of: a negativemeniscus object side negative lens element L1 with the convex surfacefacing the object side; and a positive meniscus image side positive lenselement L2 with the convex surface facing the object side. Each of theobject side negative lens element L1 and the image side positive lenselement L2 has an aspheric image side surface.

Further, in the zoom lens system according to Embodiment 1, the secondlens unit G2, in order from the object side to the image side,comprises: a positive meniscus third lens element L3 with the convexsurface facing the object side; a negative meniscus fourth lens elementL4 with the convex surface facing the object side; a negative meniscusfifth lens element L5 with the convex surface facing the object side;and a bi-convex sixth lens element L6. Among these, the third lenselement L3 and the fourth lens element L4 are cemented with each otherand thereby constitute a positive cemented lens element, while the fifthlens element L5 and the sixth lens element L6 are cemented with eachother and thereby constitute a positive cemented lens element. Further,the third lens element L3 serving as the most object side lens elementof the second lens unit G2 has an aspheric object side surface.

Further, in the zoom lens system according to Embodiment 1, the thirdlens unit G3 comprises solely a positive meniscus seventh lens elementL7 with the convex surface facing the image side. The seventh lenselement L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 1, as shown in Table 13described later, the image side positive lens element L2 constitutingthe first lens unit G1 has a notably high refractive index. Thus, in theobject side negative lens element L1, the thickness at large light beamheight is easily ensured, so that the lens thickness can be reduced.Thus, in the zoom lens system according to Embodiment 1, the overalloptical length at the time of non-use is reduced.

As shown in FIGS. 3A to 3C, in the zoom lens system according toEmbodiment 2, the first lens unit G1, in order from the object side tothe image side, comprises two lens elements consisting of: a negativemeniscus object side negative lens element L1 with the convex surfacefacing the object side; and a positive meniscus image side positive lenselement L2 with the convex surface facing the object side. Each of theobject side negative lens element L1 and the image side positive lenselement L2 has an aspheric image side surface.

In the zoom lens system according to Embodiment 2, the second lens unitG2, in order from the object side to the image side, comprises: apositive meniscus third lens element L3 with the convex surface facingthe object side; a positive meniscus fourth lens element L4 with theconvex surface facing the object side; a negative meniscus fifth lenselement L5 with the convex surface facing the object side; and abi-convex sixth lens element L6. Among these, the fourth lens element L4and the fifth lens element L5 are cemented with each other and therebyconstitute a cemented lens element. Further, the third lens element L3serving as the most object side lens element of the second lens unit G2has an aspheric object side surface.

In the zoom lens system according to Embodiment 2, the third lens unitG3 comprises solely a bi-convex seventh lens element L7. The seventhlens element L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 2, as shown in Table 13described later, the image side positive lens element L2 constitutingthe first lens unit G1 has a comparatively high refractive index. Thus,edge thickness difference is relatively easily ensured even if the lenscenter thickness is reduced, so that the lens thickness can be reduced.Thus, in the zoom lens system according to Embodiment 2, the overalloptical length at the time of non-use is reduced.

As shown in FIGS. 5A to 5C, in the zoom lens system of Embodiment 3, thefirst lens unit G1, in order from the object side to the image side,comprises two lens elements consisting of: a bi-concave object sidenegative lens element L1; and a positive meniscus image side positivelens element L2 with the convex surface facing the object side. Each ofthe object side negative lens element L1 and the image side positivelens element L2 has an aspheric image side surface.

Further, in the zoom lens system according to Embodiment 3, the secondlens unit G2, in order from the object side to the image side,comprises: a positive meniscus third lens element L3 with the convexsurface facing the object side; a negative meniscus fourth lens elementL4 with the convex surface facing the object side; a negative meniscusfifth lens element L5 with the convex surface facing the object side;and a bi-convex sixth lens element L6. Among these, the third lenselement L3 and the fourth lens element L4 are cemented with each otherand thereby constitute a positive cemented lens element, while the fifthlens element L5 and the sixth lens element L6 are cemented with eachother and thereby constitute a positive cemented lens element. Further,the third lens element L3 serving as the most object side lens elementof the second lens unit G2 has an aspheric object side surface.

Further, in the zoom lens system according to Embodiment 3, the thirdlens unit G3 comprises solely a positive meniscus seventh lens elementL7 with the convex surface facing the image side. The seventh lenselement L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 3, as shown in Table 13described later, the object side negative lens element L1 constitutingthe first lens unit G1 has a low refractive index, while the thicknessat large light beam height is small. However, the image side positivelens element L2 of the first lens unit G1 has a comparatively highrefractive index and an aspheric surface on the image side. Thus, evenwhen the thickness at large light beam height of the object sidenegative lens element L1 is small so that the compensation of distortionor curvature of field on the wide-angle limit side is insufficient, inthe entire zoom lens system according to Embodiment 3, the compensationeffect of the image side positive lens element L2 allows the image sidepositive lens element L2 to compensate sufficiently the distortion andthe curvature of field on the wide-angle limit side.

As shown in FIGS. 7A to 7C, in the zoom lens system according toEmbodiment 4, the first lens unit G1, in order from the object side tothe image side, comprises two lens elements consisting of: a negativemeniscus object side negative lens element L1 with the convex surfacefacing the object side; and a positive meniscus image side positive lenselement L2 with the convex surface facing the object side. Each of theobject side negative lens element L1 and the image side positive lenselement L2 has an aspheric image side surface.

In the zoom lens system according to Embodiment 4, the second lens unitG2, in order from the object side to the image side, comprises: abi-convex third lens element L3; a bi-concave fourth lens element L4; anegative meniscus fifth lens element L5 with the convex surface facingthe object side; and a bi-convex sixth lens element L6. Among these, thethird lens element L3 and the fourth lens element L4 are cemented witheach other and thereby constitute a positive cemented lens element,while the fifth lens element L5 and the sixth lens element L6 arecemented with each other and thereby constitute a positive cemented lenselement. Further, the third lens element L3 serving as the most objectside lens element of the second lens unit G2 has an aspheric object sidesurface.

In the zoom lens system according to Embodiment 4, the third lens unitG3 comprises solely a bi-convex seventh lens element L7. The seventhlens element L7 has an aspheric image side surface.

In the zoom lens system according to Embodiment 4, the two lens elementsL1 and L2 constituting the first lens unit G1 contact with each other ina vicinity where the light beam from the object passes. Thus, thethickness of the entire first lens unit G1 can be reduced. Further, evenwhen the object side negative lens element L1 and the image sidepositive lens element L2 of the first lens unit G1 approach with eachother so that the capability of compensating distortion is degraded, thedistortion at the wide-angle limit is compensated satisfactorily in theentire zoom lens system according to Embodiment 4 since the image sidepositive lens element L2 is a lens element having a comparatively highrefractive index as shown in Table 13 described later, and an asphericsurface on the image side.

In the zoom lens system according to Embodiments 1 to 4, the lens unitsG1 to G3 are arranged in a desired optical power construction so thatsize reduction is achieved in the entire lens system in a state thatexcellent optical performance is satisfied.

In particular, in the zoom lens system according to Embodiments 1 to 4,the first lens unit G1 is constructed from: one object side negativelens element; and one image side positive lens element with the convexsurface facing the object side. Further, the second lens unit G2 isconstructed from two sets of positive cemented lens elements eachfabricated by cementing two lens elements, or alternatively has such aconstruction that one set of cemented lens element is placed betweenpositive lens elements each arranged on the object side or the imageside. Furthermore, the third lens unit G3 is constructed from one lenselement. As such, the zoom lens system according to Embodiments 1 to 4realizes a lens system that has a small number of lens elementsconstituting each lens unit and a short overall optical length at thetime of non-use.

As described above, in the zoom lens system according to Embodiments 1to 4, the second lens unit G2 is constructed from two sets of positivecemented lens elements or alternatively has such a construction that oneset of cemented lens element is placed between positive lens elementseach arranged on the object side or the image side. Instead, the secondlens unit G2 may, in order from the object side to the image side,comprise one set of positive cemented lens element and one positive lenselement, so that a lens system can be realized that has a short overalloptical length at the time of non-use.

In the zoom lens system according to Embodiments 1 to 4, each of theobject side negative lens element and the image side positive lenselement constituting the first lens unit G1 has an aspheric surface,while the image side positive lens element has a specific refractiveindex and a specific Abbe number. Thus, the zoom lens system accordingto Embodiments 1 to 4 has excellent optical performance, for example, incompensation of curvature of field.

Conditions are described below that are to be satisfied by a zoom lenssystem like the zoom lens system according to Embodiments 1 to 4, inorder from the object side to the image side, comprises a first lensunit having negative optical power, a second lens unit having positiveoptical power, and a third lens unit having positive optical power,wherein the first lens unit is constructed from: one object sidenegative lens element; and one image side positive lens element with theconvex surface facing the object side, and wherein each of the two lenselements constituting the first lens unit has an aspheric surface. Here,a plurality of conditions to be satisfied are set forth for the zoomlens system according to each embodiment. The construction thatsatisfies all the conditions is most desirable for the zoom lens system.However, when an individual condition is satisfied, a zoom lens systemproviding the corresponding effect can be obtained.

For example, in a zoom lens system like the zoom lens system accordingto Embodiments 1 to 4, the following conditions (1) and (2) aresatisfied;n12>1.88  (1)ν12<26  (2)

where,

n12 is a refractive index of the image side positive lens element of thefirst lens unit, and

ν12 is an Abbe number of the image side positive lens element of thefirst lens unit.

The conditions (1) and (2) set forth the refractive index and the Abbenumber of the image side positive lens element constituting the firstlens unit. When these conditions (1) and (2) are satisfied, the centerthickness of the image side positive lens element becomes small, whilecurvature of field on the wide-angle limit side is suppressed withoutthe necessity of a large curvature in the image side surface, so thatedge thickness difference is easily ensured. Thus, the thickness of thefirst lens unit can be reduced. This reduces the thickness of the entirezoom lens system and hence the overall optical length at the time ofnon-use.

Here, when at least one of the following conditions (1)′ and (2)′ issatisfied, the above effect is achieved more successfully. When thefollowing condition (1)′ is satisfied, the image side positive lenselement of the first lens unit can have a large Z value (differencebetween curvature of the object side surface and curvature of the imageside surface), so that the centering of the lens becomes easier.Further, when the following condition (2)′ is satisfied, chromaticaberration generated in the first lens unit can be compensated moresatisfactorily.n12>1.95  (1)′ν12<24  (2)′

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingconditions (3), (4), (5) and (6) are satisfied;n11>1.50  (3)ν11>35  (4)n12−n11>0.10  (5)ν11−ν12>15.0  (6)

where,

n11 is a refractive index of the object side negative lens element ofthe first lens unit,

ν11 is an Abbe number of the object side negative lens element of thefirst lens unit,

n12 is the refractive index of the image side positive lens element ofthe first lens unit, and

ν12 is the Abbe number of the image side positive lens element of thefirst lens unit.

The conditions (3) and (4) set forth the refractive index and the Abbenumber of the object side negative lens element constituting the firstlens unit. The conditions (5) and (6) relate to conditions forperforming satisfactory compensation of chromatic aberration of a zoomlens system where the first lens unit is of negative-lead and hasnegative optical power while the first lens unit comprises an objectside negative lens element and an image side positive lens element. Whenthese conditions (3), (4), (5) and (6) are satisfied, a possibility isavoided that the optical axial thickness of the lens element increaseswith increasing light beam height and that when the center thickness isincreased for the purpose of improvement in manufacturability, thethickness of the entire first lens unit increases further. At the sametime, chromatic aberration can be compensated satisfactorily.

Further, when at least one of the following conditions (3)′, (4)′, (5)′and (6)′ is satisfied, the above effect is achieved more successfully.Furthermore, when at least one of the following conditions (4)″ and (6)″is satisfied, chromatic aberration generated in the first lens unit canbe compensated more satisfactorily.n11>1.75  (3)′ν11>38  (4)′65>ν11  (4)″n12−n11>0.12  (5)′ν11−ν12>17.5  (6)′45.0>ν11−ν12  (6)″

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingcondition (7) is satisfied;T1/Y<1.5  (7)

where,

T1 is a center thickness of the first lens unit, and

Y is the maximum image height.

The condition (7) sets forth the center thickness of the first lens unitin a zoom lens system where the first lens unit is of negative-lead andhas negative optical power, and hence easily becomes large. When thecondition (7) is satisfied, a possibility is avoided that the thicknessof the first lens unit increases excessively and so does the overalloptical length at the time of non-use.

Further, when the following condition (7)′ is satisfied, optical poweris imparted to the air lens in the first lens unit. Thus, thecompensation of curvature of field becomes easier on the wide-angleside.0.8<T1/Y  (7)′

Further, for example, in a zoom lens system like the zoom lens systemaccording to Embodiments 1 to 4, it is preferable that the followingcondition (8) is satisfied;(T1+T2+T3)/Y<3.5  (8)

where,

T1 is the center thickness of the first lens unit,

T2 is a center thickness of the second lens unit,

T3 is a center thickness of the third lens unit, and

Y is the maximum image height.

The condition (8) sets forth the total center thickness of the lensunits. When the condition (8) is satisfied, a possibility is avoidedthat the total thickness of the lens units increases excessively and sodoes the overall optical length at the time of non-use.

When the following condition (8)′ is satisfied, the above effect isachieved more successfully. Further, when the following condition (8)″is satisfied, the thickness of each lens unit, especially the thicknessof the first lens unit and the thickness of the second lens unit, can beensured. This permits more satisfactory compensation of curvature offield.(T1+T2+T3)/Y<3.2  (8)′2.5<(T1+T2+T3)/Y  (8)″

Here, the lens units constituting the zoom lens system of Embodiments 1to 4 are composed exclusively of refractive type lens elements thatdeflect the incident light by refraction (that is, lens elements of atype in which deflection is achieved at the interface between media eachhaving a distinct refractive index). However, the present invention isnot limited to the zoom lens system of this construction. For example,the lens units may employ diffractive type lens elements that deflectthe incident light by diffraction; refractive-diffractive hybrid typelens elements that deflect the incident light by a combination ofdiffraction and refraction; or gradient index type lens elements thatdeflect the incident light by distribution of refractive index in themedium.

Further, in the zoom lens system according to Embodiments 1 to 4, when areflecting surface may be arranged in the optical path so that theoptical path may be bent before or after the zoom lens system oralternatively in the middle. The bending position may be set uparbitrarily depending on the necessity. When the optical path is bentappropriately, thickness reduction in appearance can be achieved in acamera.

Further, the zoom lens system according to Embodiments 1 to 4 has beendescribed for the construction that a plane parallel plate P such as anoptical low-pass filter is arranged between the most image side surfaceof the third lens element G3 and the image surface S. This low-passfilter may be a birefringent type low-pass filter made of, for example,a crystal whose predetermined crystal orientation is adjusted; or aphase type low-pass filter that achieves required characteristics ofoptical cut-off frequency by diffraction. Further, this plane parallelplate P may be arranged depending on the necessity.

As described above, according to the present invention, a zoom lenssystem is obtained that compensates curvature of field satisfactorilyand that still has a reduced thickness of the first lens unit and ashort overall optical length at the time of non-use.

Embodiment 5

FIG. 9 is a schematic construction diagram of a digital still cameraaccording to Embodiment 5. In FIG. 9, the digital still cameracomprises: an imaging device including a zoom lens system 1 and an imagesensor 2 that is a CCD; a liquid crystal display monitor 3, and a body4. The employed zoom lens system 1 is the zoom lens system according toEmbodiment 1. In FIG. 9, the zoom lens system 1 comprises a first lensunit G1, a diaphragm A, a second lens unit G2, and a third lens unit G3.In the body 4, the zoom lens system 1 is arranged on the front side,while the image sensor 2 is arranged on the rear side of the zoom lenssystem 1. The liquid crystal display monitor 3 is arranged on the rearside of the body 4, while an optical image of a photographic objectacquired through the zoom lens system 1 is formed on the image surfaceS.

The lens barrel comprises a main barrel 5, a moving barrel 6, and acylindrical cam 7. When the cylindrical cam 7 is rotated, the first lensunit G1, the second lens unit G2, and the third lens unit G3 move topredetermined positions relative to the image sensor 2, so that variablemagnification can be achieved ranging from the wide-angle limit to thetelephoto limit. The third lens unit G3 is movable in the optical axisdirection by a motor for focus adjustment.

As such, when the zoom lens system according to Embodiment 1 is employedin a digital still camera, a small digital still camera is obtained thathas a high resolution and high capability of compensating the curvatureof field and that has a short overall optical length at the time ofnon-use. Here, in the digital still camera shown in FIG. 9, any one ofthe zoom lens systems according to Embodiments 2 to 4 may be employed inplace of the zoom lens system according to Embodiment 1. Further, theoptical system of the digital still camera shown in FIG. 9 may beapplied to a digital video camera for moving images. In this case,moving images with high resolution can be acquired in addition to stillimages.

An imaging device comprising a zoom lens system according to Embodiments1 to 4 described above and an image sensor such as a CCD or a CMOS maybe applied to a mobile telephone, a PDA (Personal Digital Assistance), asurveillance camera in a surveillance system, a Web camera, avehicle-mounted camera or the like.

Hereinafter, numerical examples which are actual implementations of thezoom lens systems according to Embodiments 1 to 4 will be described. Inthe numerical examples, the units of the length in the tables are all“mm”. Moreover, r is the radius of curvature, d is the axial distance,nd is the refractive index to the d-line, and νd is the Abbe number tothe d-line. In the numerical examples, the surfaces marked with * areaspherical surfaces, and the sag z of the aspherical surface is definedby the following expression:

$z = {\frac{{ch}^{2}}{1 + \sqrt{\left\{ {1 - {\left( {1 + k} \right)c^{2}h^{2}}} \right\}}} + {A\; h^{4}} + {Bh}^{6} + {Ch}^{8} + {Dh}^{10} + {Eh}^{12}}$Here, h is the height from the optical axis, c is the curvature, k isthe conic constant, and A, B, C, D and E are the fourth-order,sixth-order, eighth-order, tenth-order and twelfth-order asphericalcoefficients, respectively.

Example 1

A zoom lens system of Example 1 corresponds to Embodiment 1 shown inFIGS. 1A to 1C. Table 1 shows the lens data of the zoom lens system ofExample 1. Table 2 shows the aspherical data. Table 3 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d11 and d13, when the shootingdistance is infinity.

TABLE 1 Lens Lens unit element Surface r d nd νd G1 L1 1 51.824 1.1001.805 41.0 *2 6.287 2.433 L2 3 12.887 1.400 2.400 17.0 *4 17.651Variable Diaphragm 5 ∞ 0.300 G2 L3 *6 4.617 1.900 1.805 41.0 L4 7 30.1910.500 1.717 29.5 8 4.069 0.600 L5 9 19.289 0.600 1.620 36.3 L6 10 4.4131.200 1.589 61.3 11 −12.764 Variable G3 L7 12 −237.873 1.100 1.665 55.2*13 −15.297 Variable P 14 ∞ 0.900 1.517 64.2 15 ∞ 0.870

TABLE 2 Surface k A B C D E 2 −3.612E−01   7.170E−06 −3.185E−06−1.903E−09  −1.340E−09 0.000E+00 4 0.000E+00 −9.258E−05  1.154E−060.000E+00  0.000E+00 0.000E+00 6 0.000E+00 −5.397E−04 −1.839E−051.169E−07 −3.148E−08 0.000E+00 13 0.000E+00  4.422E−04 −5.274E−056.216E−06 −3.426E−07 7.090E−09

TABLE 3 Axial Wide-angle Middle Telephoto distance limit position limitd4 14.79 8.07 2.37 d11 2.70 9.09 19.06 d13 6.10 4.47 2.64 f 5.36 8.8116.71 F-number 2.98 3.86 5.66 2ω 70.01 44.60 23.97 L 36.50 34.53 36.96

Example 2

A zoom lens system of Example 2 corresponds to Embodiment 2 shown inFIGS. 3A to 3C. Table 4 shows the lens data of the zoom lens system ofExample 2. Table 5 shows the aspherical data. Table 6 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d12 and d14, when the shootingdistance is infinity.

TABLE 4 Lens Lens unit element Surface r d nd νd G1 L1 1 97.238 1.3001.878 38.2 *2 6.116 2.181 L2 3 15.696 1.778 1.996 20.5 *4 62.842Variable Diaphragm 5 ∞ 0.300 G2 L3 *6 4.711 1.500 1.804 40.8 7 20.9310.300 L4 8 8.092 0.800 1.697 55.5 L5 9 52.433 0.400 1.805 25.5 10 3.5210.419 L6 11 24.775 0.993 1.697 55.5 12 −24.775 Variable G3 L7 13 33.5511.438 1.518 70.3 *14 −15.270 Variable P 15 ∞ 0.900 1.517 64.2 16 ∞ 0.870

TABLE 5 Surface k A B C D E 2 −7.285E−01  1.632E−04 −1.177E−05 3.548E−07 −1.538E−09  0.000E+00 4  0.000E+00 −1.619E−04  7.984E−06−2.917E−07 2.109E−09 0.000E+00 6 −1.425E−01 −4.078E−04  1.138E−05−7.290E−06 7.546E−07 0.000E+00 14  0.000E+00  2.119E−04 −1.102E−05 1.904E−07 1.254E−08 −4.126E−10 

TABLE 6 Axial Wide-angle Middle Telephoto distance limit position limitd4 16.24 6.86 1.68 d12 2.70 7.90 17.30 d14 4.39 3.93 2.60 f 4.83 8.7916.50 F-number 2.99 3.96 5.91 2ω 75.75 45.08 24.67 L 36.58 31.92 34.84

Example 3

A zoom lens system of Example 3 corresponds to Embodiment 3 shown inFIGS. 5A to 5C. Table 7 shows the lens data of the zoom lens system ofExample 3. Table 8 shows the aspherical data. Table 9 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d11 and d13, when the shootingdistance is infinity.

TABLE 7 Lens Lens unit element Surface r d nd νd G1 L1 1 −84.423 1.1001.514 63.3 *2 5.404 2.309 L2 3 14.387 1.600 1.900 24.0 *4 26.719Variable Diaphragm 5 ∞ 0.300 G2 L3 *6 4.964 1.900 1.805 41.0 L4 7144.593 0.500 1.717 29.5 8 4.496 0.600 L5 9 43.557 0.600 1.620 36.3 L610 6.034 1.200 1.589 61.3 11 −10.835 Variable G3 L7 12 −237.873 1.1001.665 55.2 *13 −14.202 Variable P 14 ∞ 0.900 1.517 64.2 15 ∞ 0.870

TABLE 8 Surface k A B C D E 2 −6.222E−01   2.697E−04 −9.608E−06−1.996E−07  4.805E−09 0.000E+00 4 0.000E+00 −2.732E−04  9.958E−06−1.370E−07  0.000E+00 0.000E+00 6 0.000E+00 −5.198E−04 −1.810E−05 1.721E−06 −1.330E−07 0.000E+00 13 0.000E+00  4.072E−04 −5.130E−05 6.943E−06 −4.141E−07 9.128E−09

TABLE 9 Axial Wide-angle Middle Telephoto distance limit position limitd4 14.38 8.04 2.03 d11 2.70 9.14 19.40 d13 6.45 4.83 2.60 f 5.57 8.8216.70 F-number 2.94 3.77 5.56 2ω 67.94 43.94 23.71 L 35.61 34.10 36.11

Example 4

A zoom lens system of Example 4 corresponds to Embodiment 4 shown inFIGS. 7A to 7C. Table 10 shows the lens data of the zoom lens system ofExample 4. Table 11 shows the aspherical data. Table 12 shows the focallength f, the F-number, the view angle 2ω, the overall optical length L,and the variable axial distance data d4, d11 and d13, when the shootingdistance is infinity.

TABLE 10 Lens Lens unit element Surface r d nd νd G1 L1 1 96.707 1.1001.878 38.2 *2 5.757 1.219 L2 3 9.382 1.778 1.996 20.5 *4 22.204 VariableDiaphragm 5 ∞ 0.300 G2 L3 *6 4.225 1.500 1.805 41.0 L4 7 −50.000 0.6001.717 29.5 8 3.642 0.600 L5 9 15.017 0.600 1.620 36.3 L6 10 5.586 1.5001.589 61.3 11 −16.364 Variable G3 L7 12 100.000 1.100 1.665 55.2 *13−15.520 Variable P 14 ∞ 0.900 1.517 64.2 15 ∞ 0.870

TABLE 11 Surface k A B C D E 2 −7.802E−01  5.551E−05  1.727E−07−1.998E−07  6.955E−09 0.000E+00 4  0.000E+00 −4.846E−05  3.420E−06 4.006E−08 −4.178E−09 0.000E+00 6  0.000E+00 −6.412E−04 −4.796E−05 6.036E−06 −6.470E−07 0.000E+00 13  0.000E+00  7.442E−04 −9.347E−05 9.894E−06 −5.255E−07 1.088E−08

TABLE 12 Axial Wide-angle Middle Telephoto distance limit position limitd4 16.77 7.10 2.17 d11 2.70 8.24 17.94 d13 4.97 4.43 2.64 f 4.75 8.8116.43 F-number 2.78 3.66 5.40 2ω 76.50 44.06 24.09 L 36.51 31.84 34.82

Table 13 shows values corresponding to the conditions in Examples 1 to4.

TABLE 13 Example Condition 1 2 3 4 (1) n12 2.40 2.00 1.90 2.00 (2) ν 1217.0 20.5 24.0 20.5 (3) n11 1.80 1.88 1.51 1.88 (4) ν 11 40.95 38.2063.28 38.20 (5) n12 − n11 0.60 0.12 0.39 0.12 (6) ν 11 − ν 12 23.9517.67 39.28 17.67 (7) T1/Y 1.37 1.46 1.39 1.14 (8) (T1 + T2 + T3)/Y 3.013.09 3.03 2.78 T1 4.93 5.26 5.01 4.10 T2 4.80 4.41 4.80 4.80 T3 1.101.44 1.10 1.10 T1 + T2 + T3 10.83 11.11 10.91 10.00 Y 3.60 3.60 3.603.60

FIGS. 2A to 2I are longitudinal aberration diagrams of a zoom lenssystem according to Example 1. FIGS. 4A to 4I are longitudinalaberration diagrams of a zoom lens system according to Example 2. FIGS.6A to 6I are longitudinal aberration diagrams of a zoom lens systemaccording to Example 3. FIGS. 8A to 8I are longitudinal aberrationdiagrams of a zoom lens system according to Example 4.

FIGS. 2A to 2C, 4A to 4C, 6A to 6C, and 8A to 8C show the longitudinalaberration at the wide-angle limit. FIGS. 2D to 2F, 4D to 4F, 6D to 6F,and 8D to 8F show the longitudinal aberration at an approximate middleposition. FIGS. 2G to 2I, 4G to 4I, 6G to 6I, and 8G to 8I show thelongitudinal aberration at the telephoto limit. FIGS. 2A, 2D, 2G, 4A,4D, 4G, 6A, 6D, 6G, 8A, 8D and 8G are spherical aberration diagrams.FIGS. 2B, 2E, 2H, 4B, 4E, 4H, 6B, 6E, 6H, 8B, 8E and 8H are astigmatismdiagrams. FIGS. 2C, 2F, 2I, 4C, 4F, 4I, 6C, 6F, 6I, 8C, 8F and 8I aredistortion diagrams. In each spherical aberration diagram, the verticalaxis indicates the F-number, and the solid line, the short dash line andthe long dash line indicate the characteristics to the d-line, theF-line and the C-line, respectively. In each astigmatism diagram, thevertical axis indicates the half view angle ω, and the solid line andthe dash line indicate the characteristics to the sagittal image plane(in each Fig., indicated as “s”) and the meridional image plane (in eachFig., indicated as “m”), respectively. In each distortion diagram, thevertical axis indicates the half view angle ω.

The zoom lens system according to the present invention is applicable toa camera such as a digital still camera, a digital video camera, amobile telephone, a PDA (Personal Digital Assistance), a surveillancecamera in a surveillance system, a Web camera or a vehicle-mountedcamera. In particular, the present zoom lens system is suitable for acamera such as a digital still camera or a digital video camerarequiring high image quality.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention, theyshould be construed as being included therein.

1. A zoom lens system, in order from an object side to an image side,comprising: a first lens unit having negative optical power and movingalong an optical axis during zooming, the first lens unit, in order fromthe object side to the image side, having a negative lens element and apositive lens element with a convex surface facing the object side; asecond lens unit having positive optical power and moving along anoptical axis during zooming; and a third lens unit having positiveoptical power and moving along an optical axis during zooming, whereinthe second lens unit has four lens elements, the negative lens elementof the first lens unit has an aspheric surface, and the followingcondition (1) is satisfied:n12>1.88  (1) (here, 2ω_(W)≧67.94) where, n12 is a refractive index ofthe positive lens element of the first lens unit, and 2ω_(W) is a viewangle at a wide-angle limit.
 2. The zoom lens system of claim 1, whereinthe second lens unit has at least one cemented lens element.
 3. The zoomlens system of claim 2, wherein the second lens unit has two sets ofpositive cemented lens elements.
 4. The zoom lens system of claim 2,wherein the second lens unit has, in order from the object side to theimage side, a positive lens element, a cemented lens element and a lenselement.
 5. An imaging device capable of converting an optical image ofa photographic object into an electric image signal and then outputtingthe signal, comprising: a zoom lens system that forms the optical imageof the photographic object with a variable magnification; and an imagesensor that converts the optical image of the photographic object formedby the zoom lens system into the electric image signal, wherein the zoomlens system, in order from an object side serving as the photographicobject side to an image side, comprises: a first lens unit havingnegative optical power and moving along an optical axis during zooming,the first lens unit, in order from the object side to the image side,having a negative lens element and a positive lens element with a convexsurface facing the object side; a second lens unit having positiveoptical power and moving along an optical axis during zooming; and athird lens unit having positive optical power and moving along anoptical axis during zooming, wherein the second lens unit has four lenselements, the negative lens element of the first lens unit has anaspheric surface, and the following condition (1) is satisfied:n12>1.88  (1) (here, 2ω_(W)≧67.94) where, n12 is a refractive index ofthe positive lens element of the first lens unit, and 2ω_(W) is a viewangle at a wide-angle limit.
 6. A camera capable of shooting aphotographic object and then outputting an image of the photographicobject as an electric image signal, comprising: an imaging device havinga zoom lens system that forms the optical image of the photographicobject with a variable magnification, and an image sensor that convertsthe optical image of the photographic object formed by the zoom lenssystem into the electric image signal, wherein the zoom lens system, inorder from an object side serving as the photographic object side to animage side, comprises: a first lens unit having negative optical powerand moving along an optical axis during zooming, the first lens unit, inorder from the object side to the image side, having a negative lenselement and a positive lens element with a convex surface facing theobject side; a second lens unit having positive optical power and movingalong an optical axis during zooming; and a third lens unit havingpositive optical power and moving along an optical axis during zooming,wherein the second lens unit has four lens elements, the negative lenselement of the first lens unit has an aspheric surface, and thefollowing condition (1) is satisfied:n12>1.88  (1) (here, 2ω_(W)≧67.94) where, n12 is a refractive index ofthe positive lens element of the first lens unit, and 2ω_(W) is a viewangle at a wide-angle limit.
 7. A zoom lens system, in order from anobject side to an image side, comprising: a first lens unit havingnegative optical power and moving along an optical axis during zooming,the first lens unit, in order from the object side to the image side,having a negative lens element and a positive lens element with a convexsurface facing the object side; a second lens unit having positiveoptical power and moving along an optical axis during zooming; and athird lens unit having positive optical power and moving along anoptical axis during zooming, wherein the second lens unit has at leastthree lens elements, the negative lens element of the first lens unithas an aspheric surface, the third lens unit monotonically moves to theimage side during zooming, and the following condition (1) is satisfied:n12>1.88  (1) (here, 2ω_(W)≧67.94) where, n12 is a refractive index ofthe positive lens element of the first lens unit, and 2ω_(W) is a viewangle at a wide-angle limit.
 8. The zoom lens system of claim 7, whereinthe second lens unit has at least one cemented lens element.
 9. The zoomlens system of claim 8, wherein the second lens unit has two sets ofpositive cemented lens elements.
 10. The zoom lens system of claim 8,wherein the second lens unit has, in order from the object side to theimage side, a positive lens element, a cemented lens element and a lenselement.
 11. An imaging device capable of converting an optical image ofa photographic object into an electric image signal and then outputtingthe signal, comprising: a zoom lens system that forms the optical imageof the photographic object with a variable magnification; and an imagesensor that converts the optical image of the photographic object formedby the zoom lens system into the electric image signal, wherein: thezoom lens system, in order from an object side serving as thephotographic object side to an image side, comprises: a first lens unithaving negative optical power and moving along an optical axis duringzooming, the first lens unit, in order from the object side to the imageside, having a negative lens element and a positive lens element with aconvex surface facing the object side; a second lens unit havingpositive optical power and moving along an optical axis during zooming;and a third lens unit having positive optical power and moving along anoptical axis during zooming, wherein: the second lens unit has at leastthree lens elements, the negative lens element of the first lens unithas an aspheric surface, the third lens unit monotonically moves to theimage side during zooming, and the following condition (1) is satisfied:n12>1.88  (1) (here, 2ω_(W)≧67.94) where, n12 is a refractive index ofthe positive lens element of the first lens unit, and 2ω_(W) is a viewangle at a wide-angle limit.
 12. A camera capable of shooting aphotographic object and then outputting an image of the photographicobject as an electric image signal, comprising: an imaging device havinga zoom lens system that forms the optical image of the photographicobject with a variable magnification, and an image sensor that convertsthe optical image of the photographic object formed by the zoom lenssystem into the electric image signal, wherein: the zoom lens system, inorder from an object side serving as the photographic object side to animage side, comprises: a first lens unit having negative optical powerand moving along an optical axis during zooming, the first lens unit, inorder from the object side to the image side, having a negative lenselement and a positive lens element with a convex surface facing theobject side; a second lens unit having positive optical power and movingalong an optical axis during zooming; and a third lens unit havingpositive optical power and moving along an optical axis during zooming,wherein: the second lens unit has at least three lens elements, thenegative lens element of the first lens unit has an aspheric surface,the third lens unit monotonically moves to the image side duringzooming, and the following condition (1) is satisfied:n12>1.88  (1) (here, 2ω_(W)≧67.94) where, n12 is a refractive index ofthe positive lens element of the first lens unit, and 2ω_(W) is a viewangle at a wide-angle limit.